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><A
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></A
>8.4. Operator Precedence</H1
><P
><A
NAME="OPPRECEDENCE1"
></A
></P
><P
>        In a script, operations execute in order of
        <I
CLASS="FIRSTTERM"
>precedence</I
>: the higher precedence operations
	execute <EM
>before</EM
> the lower precedence ones.
	  <A
NAME="AEN4290"
HREF="#FTN.AEN4290"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
      </P
><DIV
CLASS="TABLE"
><A
NAME="AEN4294"
></A
><P
><B
>Table 8-1. Operator Precedence</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><THEAD
><TR
><TH
ALIGN="LEFT"
VALIGN="TOP"
>Operator</TH
><TH
ALIGN="LEFT"
VALIGN="TOP"
>Meaning</TH
><TH
ALIGN="LEFT"
VALIGN="TOP"
>Comments</TH
></TR
></THEAD
><TBODY
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><B
CLASS="COMMAND"
>HIGHEST PRECEDENCE</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>var++ var--</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>post-increment, post-decrement</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="assortedtips.html#CSTYLE"
>C-style</A
> operators</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>++var --var</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>pre-increment, pre-decrement</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>! ~</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="special-chars.html#NOTREF"
>negation</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>logical / bitwise, inverts sense of following
	        operator</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>**</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#EXPONENTIATIONREF"
>exponentiation</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#AROPS1"
>arithmetic
	        operation</A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>* / %</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>multiplication, division, modulo</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>arithmetic operation</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>+ -</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>addition, subtraction</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>arithmetic operation</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>&#60;&#60; &#62;&#62;</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>left, right shift</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#BITWSOPS1"
>bitwise</A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>-z -n</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><I
CLASS="FIRSTTERM"
>unary</I
> comparison</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>string is/is-not <A
HREF="comparison-ops.html#STRINGNULL"
>null</A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>-e -f -t -x, etc.</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><I
CLASS="FIRSTTERM"
>unary</I
> comparison</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="fto.html"
>file-test</A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>&#60; -lt &#62; -gt &#60;= -le &#62;= -ge</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><I
CLASS="FIRSTTERM"
>compound</I
> comparison</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>string and integer</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>-nt -ot -ef</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><I
CLASS="FIRSTTERM"
>compound</I
> comparison</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>file-test</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>== -eq <A
HREF="comparison-ops.html#NOTEQUAL"
>!=</A
>
	       -ne</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>equality / inequality</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>test operators, string and integer</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>&#38;</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>AND</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>bitwise</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>^</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>XOR</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><I
CLASS="FIRSTTERM"
>exclusive</I
> OR, bitwise</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>|</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>OR</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>bitwise</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>&#38;&#38; -a</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>AND</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#LOGOPS1"
>logical</A
>,
	        <I
CLASS="FIRSTTERM"
>compound</I
>
	        comparison</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>|| -o</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>OR</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>logical, <I
CLASS="FIRSTTERM"
>compound</I
>
	        comparison</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>?:</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="special-chars.html#CSTRINARY"
>trinary
	        operator</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>C-style</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>=</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="varassignment.html#EQREF"
>assignment</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>(do not confuse with equality
	        <I
CLASS="FIRSTTERM"
>test</I
>)</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>*= /= %= += -= &#60;&#60;= &#62;&#62;= &#38;=</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#ARITHOPSCOMB"
>combination
	        assignment</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>times-equal, divide-equal, mod-equal, etc.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
>,</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><A
HREF="ops.html#COMMAOP"
>comma</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>links a sequence of operations</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="OPTION"
></TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><B
CLASS="COMMAND"
>LOWEST PRECEDENCE</B
></TD
></TR
></TBODY
></TABLE
></DIV
><P
>In practice, all you really need to remember is the
      following:</P
><P
></P
><UL
><LI
><P
>The <SPAN
CLASS="QUOTE"
>"My Dear Aunt Sally"</SPAN
> mantra (<EM
>multiply,
	  divide, add, subtract</EM
>) for the familiar <A
HREF="ops.html#AROPS1"
>arithmetic operations</A
>.</P
></LI
><LI
><P
>The <I
CLASS="FIRSTTERM"
>compound</I
> logical operators,
	  <B
CLASS="COMMAND"
>&#38;&#38;</B
>, <B
CLASS="COMMAND"
>||</B
>, <B
CLASS="COMMAND"
>-a</B
>,
	  and <B
CLASS="COMMAND"
>-o</B
> have low precedence.</P
></LI
><LI
><P
>The order of evaluation of equal-precedence operators is
	  usually <I
CLASS="FIRSTTERM"
>left-to-right</I
>.</P
></LI
></UL
><P
>Now, let's utilize our knowledge of operator precedence to
	  analyze a couple of lines from the
	  <TT
CLASS="FILENAME"
>/etc/init.d/functions file</TT
>, as found in
	  the <I
CLASS="FIRSTTERM"
>Fedora Core</I
> Linux distro.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>while [ -n "$remaining" -a "$retry" -gt 0 ]; do

# This looks rather daunting at first glance.


# Separate the conditions:
while [ -n "$remaining" -a "$retry" -gt 0 ]; do
#       --condition 1-- ^^ --condition 2-

#  If variable "$remaining" is not zero length
#+      AND (-a)
#+ variable "$retry" is greater-than zero
#+ then
#+ the [ expresion-within-condition-brackets ] returns success (0)
#+ and the while-loop executes an iteration.
#  ==============================================================
#  Evaluate "condition 1" and "condition 2" ***before***
#+ ANDing them. Why? Because the AND (-a) has a lower precedence
#+ than the -n and -gt operators,
#+ and therefore gets evaluated *last*.

#################################################################

if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then


# Again, separate the conditions:
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
#    --condition 1--------- ^^ --condition 2-----

#  If file "/etc/sysconfig/i18n" exists
#+      AND (-a)
#+ variable $NOLOCALE is zero length
#+ then
#+ the [ test-expresion-within-condition-brackets ] returns success (0)
#+ and the commands following execute.
#
#  As before, the AND (-a) gets evaluated *last*
#+ because it has the lowest precedence of the operators within
#+ the test brackets.
#  ==============================================================
#  Note:
#  ${NOLOCALE:-} is a parameter expansion that seems redundant.
#  But, if $NOLOCALE has not been declared, it gets set to *null*,
#+ in effect declaring it.
#  This makes a difference in some contexts.</PRE
></FONT
></TD
></TR
></TABLE
></P
><DIV
CLASS="TIP"
><P
></P
><TABLE
CLASS="TIP"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/tip.gif"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>To avoid confusion or error in a complex sequence of test
        operators, break up the sequence into bracketed sections.
      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>if [ "$v1" -gt "$v2"  -o  "$v1" -lt "$v2"  -a  -e "$filename" ]
# Unclear what's going on here...

if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] &#38;&#38; [[ -e "$filename" ]]
# Much better -- the condition tests are grouped in logical sections.</PRE
></FONT
></TD
></TR
></TABLE
>
      </P
></TD
></TR
></TABLE
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
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VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN4290"
HREF="opprecedence.html#AEN4290"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><I
CLASS="FIRSTTERM"
>Precedence</I
>, in this context,
	    has approximately the same meaning as
	    <I
CLASS="FIRSTTERM"
>priority</I
></P
></TD
></TR
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